Method of and apparatus for depositing coatings on surfaces



Feb. 3, 1959 D. GABOR ET AL METHOD OF AND APPARATUS FOR DEPOSITING COATINGS ON SURFACES 2 Sheets-Sheet 1 Filed Oct. 2, 1957 Feb. 3, 1959 D. GABOR ET AL 7 METHOD OF AND APPARATUS FOR DEPOSITING. COATINGS ON SURFACES Filed Oct. 2, 1957 2 Sheets-Sheet 2 FIG. 4.

United States Patent METHOD or AND APPARATUS FOR DEPOSITING COATINGS 0N SURFACES Dennis Gabor, London, and Peter Gabor Kalman, Edgware, England, assignorsto National Research Development Corporation, London, England, a British corporation Application October 2, 1957, Serial No. 687,637

Claims priority, application Great Britain October 17, 1956 9 Claims. (Cl. 117-33) This invention relates to methods of and apparatus for the deposition of coatings on surfaces and particularly to the application to surfaces of materials in powdered form in accurately reproduced-patterns. The invention is primarily concerned with the production of screens for cathode ray tubes and with the problem of producing interlaced" patterns of phosphors having dilferent colour luminescence, or in other words, mosaics, for the purpose of coloured displays, particularly of television pictures. Additionally the invention is directed to the production of such a mosaic distributed in predetermined relationship to a shadow-mask by which the colour presentation is controlled. A

In the specification of U. S. A. Patent No. 2,795,729 there is described a cathode ray tube of flat configuration in which, in some embodiments, a shadow mask is used to control the electron beam for the purpose of colour television presentation. The present invention is particularly suitedto the production of screens in tubes of that kind because of the small spacing between the shadow mask and the phosphor screen.

The specification of copending patent application Serial No. 480,407, and now U. S. Patent No. 2,794,655, describes a materialmand method of making same, which may be used as the shadow mask of a colour television tubeof the kind described in U. S. A. Patent No. 2,795, 729. This materialris suitablefor'use as a shadow mask or stencil in performing the present invention. A fragment of such a material on a greatly enlarged scale, and attached to a glass support is shown in perspective, in Fig; 1a of the accompanying drawings. Before proceeding to describe themethod according to this invention, however, a simplified form of shadow mask material and method of making same will bedesc'ribed. When it is possible-to use a unipotential shadow mask, it is unnecessary-to have the spaced conductors insulated from one anotherso-that the insulating fibres x introduced in the transverse direction in the material of Fig. 1a are no longer required. Instead, the material may take the form illustrated in Fig. 1b of the accompanying drawings; It will-be appreciated that the mask materials of Figs. la and lbwhen destined foruse incathode ray tubes for colour. television, say, are; in practice, of very small dimensions. The conductors 1 may be of the order of 0.019 inch wide separated by gaps of 0.006 inch,- the ribs 2. being of 0.024. inch height. This invention and the following description thereof should therefore be considered as mainly applicable to the use of structures of this order of size, although of course it is not limited to such dimensions.

Careful study of Fig. 1b will show that the longitudinal conductors 1 are connected and held together by cross ribs 2 formed by folds in the material. This structure may be achieved quite simply by starting with a metal foil onwhichis printed in a suitable resist the pattern of the conductors, leaving the spaces between them uncovered,- folding thefoil to form the ribs 2 and then 2,872,339 eatented Feb. s, 1959 ice etching away the material where it is not protected by the resist in which the pattern was printed. In order to avoid etching away the ribs, not to mention the wanted parts of the pattern, the back surface of the foil may be coated all over with a resist. It is not absolutely necessary, however, to go to these lengths. Instead, the foil used as starting material may have twice the thickness required in the final product and is printed on one side only with the line pattern constituting the grid structure of the shadow mask. The foil is then crimped to form the ribs 2. The crimping presses the inner surfaces of the folds forming the ribs 2 so tightly together that the entry of etching medium into the crease is prevented so that these surfaces are entirely protected from etching. Etching is then carried out from both sides of the material so that the material is etched entirely away where the etching agent has access to both surfaces. Where the line pattern has been printed on, however, and at the ribs, the etching agent has access only to one side of the foil so that in these regious it is etched only half through.

Another variation of'thc method comprises printing the foil with the line structure on both sides, the printing on one side being accurately in register with that on the other. Etching can then take place from both sides, and in the spaces between the printed lines the material will the foil between the printed lines of the resist.

ribs the full thickness of the foil will be maintained.

However it is produced, the shadow mask material. above described may be mounted directly upon the glass surface on which the powder pattern is to be produced by the process according to this invention, the ribs 2 provid-- ing the required spacing between the mask and the sur-- face. In making colour screens for cathode ray tubeswhere the mask is required to be permanently fixed upon: the surface, it may be stuck down with a suitable agent: such as a silicone adhesive, or fused on with glass solder.. In these circumstances, it is possible to stick the mask. on the glass surface either before or after the etching: process is carried out. If it is stuck on before etching,. this has the advantage that the structure is firmly supported at all times and it can therefore be made of' thinner material or finer in structure than if it is required to be handled between the etching process and its application to the glass surface. On the other hand, it may be necessary if the crimped material is stuck on to the glass before etching to take special precautions to ensure that the etching agent reaches both sides'of the foil uniformly and is not kept away by trapped air. It is usually possible to achieve this result by the application of suction to the ends of the tunnels'formed, between the ribs, between the foil and the glass;

The invention may be carried out starting with a structure in which the .shadow mask is mounted upon the glass surface as above described, and such an arrangement is preferred, but it is also, of course, possibleto carry out the process by bringing the surface to be coated into suitable, spaced relationship to a mask or stencil by which the coating pattern is to be produced only temporarily withinthe coating chamber.

According to the invention a method of applying a coating of finely divided material to a surface through a'stencil supported in spaced relation thereto in such a manner that the coated area conforms to a projection of theconfigurations of the stencil, on the surface, comprises enclosing thesurface'xtobe coated, with the stencil posi tioned aboveit, in the said spaced relation'in 'achambet' sufficiently tightly closed to ensure that it contains a column of stagnant air, setting up a cloud of the finely divided material above said stencil and allowing the finely divided material to settle through the column of stagnant air, through the stencil and on to the surface by gravity.

According to a feature of the invention a method of coatingja surface with a mosaic comprising a plurality of different finely divided materials in separate interlaced patterns, comprises settling each deposit in turn with the surface and the stencil presented in a different attitude towards the vertical for each step in the formation of the mosaic,

According to the invention in another aspect there is provided apparatus for applying a coating of finely divided material to a surface through a stencil supported in spaced relation thereto, comprising an enclosed chamber, means within said chamber near its lower end for supporting the surface to be coated and said stencil in a desired attitude towards the vertical and means above said supporting means for setting up a cloud of said finely divided material. According to a feature of the invention in this aspect the means for setting up the cloud of finely divided material comprises a plurality of discharge openings or nozzles mounted in facing relation in such a fashion that air blasts issuing substantially simultaneously therefrom tend to neutralize one anothers momentum and localise the turbulence set up thereby. Alternatively, or in addition, such discharge openings or nozzles are so disposed and/ or directed in relation to the interior of the chamber that turbulence or wave motions set up by the discharge of air blasts therethrough are prevented from penetrating downwardly through the chamber snfficiently to disturb the settling of the finely divided material through the stencil.

In the description which follows the method according to the invention will be described in its application to the production of three-colour mosaic screens for cathode ray tubes, particularly for colour television. It will accordingly be assumed that a structure of the kind above described in which a shadow mask or stencil is attached to the glass surface to be coated will be employed. Reference will be made to the accompanying drawings in which:

Figs. 1a and 1b, which have already been referred to, show enlarged perspective views of fragments of shadow mask materials.

Fig. 2 is an enlarged fragmentary cross-sectional view of a shadow mask mounted in spaced relation to a surface to be coated, showing areas of phosphor of different colour values.

Fig. 3 is a diagrammatic representation of an apparatus which may be used in carrying out the invention, and

Pig. 4 represents a nozzle which may be used in a modified form of apparatus.

In order to deposit powder through a stencil on to a glass surface retaining in the deposit the configuration of the stencil in accurate detail, it has been found that steps must be taken to avoid any disturbing currents of air at or near the surface which will tend to carry the powder particles away from their desired locations. Thus, it is found that if it is attempted to blow powder through a stencil on to a surface, the moving air on impact with the surface sets up transverse air currents which carry the powder particles off from the initial direction of the air blast to regions behind the stencil where they are not wanted. Attempts to project the particles towards the stencil by impulsive movements of a diaphragm have proved unsuccessful probably because the shock wave set up in the air had the effect of influencing the movements of the powder particles so that again they reached parts of the surface behind the stencil where they were not wanted.

Fig. 2 shows in cross section and again on a large scale the kind of structure the invention is designed to deal with, the conductors 1 forming the shadow mask being supported in spaced relationship to the glass surface 3 which is part of a cathode ray tube end cap. The areas marked R, G and B represent in cross section three strips of phosphor forming the line structure of the cathode ray tube colour screen for the production of which the invention is to be employed. The three ditferent colour strips are, in operation of the tube, to be selectively bombarded by electron beams penetrating the shadow mask formed by conductors 1 at the three representative angles indicated by the three beams r, g and b. The angles of these three beams are predetermined by the electron optical properties of the cathode ray tube and the problem to which the present invention is directed is that of depositing on the glass surface the three sets of phosphor strips corresponding to red, green and blue in the colour screen mosaic. The process involves settling the powders through the gaps between the conductors 1 by gravity; that is to say the powders are to be allowed to fall vertically downwards through the shadow mask. It has been found that if the precautions mentioned herein are taken it is possible to perform this deposition with considerable accuracy, the assembly comprising'the glass screen 3 and the shadow mask 1 being disposed at the appropriate angle to the vertical for the disposition of each component colour of the mosaic.

The angles of incidence of the electron beams r, g and b are as shown, somewhat oblique to the screen 3, and this may have the disadvantage that if the screen with the shadow mask mounted on it is presented to the falling particles in the same aspect as they are presented to the electron beam the surface 3 is too sloping and there may be a tendency for particles to roll down or bounce off the surface into wrong positions.

Fig. 2 illustrates how-if this difficulty is met it may be overcome by depositing the various phosphor strips through adjacent slots in the shadow mask rather than through the same slots as those through which the electron bombardment will take place when the tube is in op eration. Thus the strip R, which will in operation be bombarded by an electron beam penetrating the shadow mask along the trajectory indicated by arrow r, may be deposited by powder falling through the adjacent slot as indicated by the broken lines r, while the strip G bombarded by the beam indicated at g may be deposited in the direction indicated by the broken lines g. The directions of deposition 1" and g are much more nearly perpendicular to the surface 3 than are the corresponding electron beams, so that when the shadow mask and the glass surface are positioned for the deposition, it will be in a more clearly horizontal attitude than it would have been in order to effect deposition along the same path as subsequent electron bombardments.

Fig. 3 shows the apparatus which may be used for carrying out the invention. This comprises a tall conveniently cylindrical chamber 10, near the lower end of which the assembly comprising the glass screen, with the shadow mask attached to it, is placed in the proper attitude towards the vertical for particles falling vertically -to alight in the desired positions on the glass screen through the apertures in the shadow mask. A guide 11, accessible through an aperture in the side of the chamber is indicated to suggest the means for properly positioning the stencil/screen assembly. The lower half of the cylinder 10 merely provides a column of stagnant air through which the powder particles drop so as to be deposited through the shadow mask on to the glass surface. In the upper part of the chamber there are provided means for launching the powder. It is necessary to launchthe powder in such a way that the following criteria are observed:

(a) The column of air in the lower part of the chamber must not be disturbed. This involves avoiding setting up waves which will be transmitted through the chamber, reflected from the top closure and so on.

(b) The powder must be reasonably uniformly distributed' over the cross section of the chamber or at least over that part of it over which the deposition is to occur.

(0) The powder particles must be dispersed. It is important to avoid as far as possible groups of particles, sticking together by electrostatic attraction or otherwise being launched as such since they tend to behave as single large particles with disadvantageous results .as will appear hereinafter.

These criteria are met by launching the powder in such a way that a substantially stationary cloud is formed in the upper part of the chamber, this cloud being left to settle in its own time down through the chamber. One

' principle which may be used to launch such a cloud is to employ a number of nozzles through which puffs of air carrying the powder are caused to emerge simultaneously. If the nozzles are arranged facing one another and some care is taken to balance the air blasts one against the other it can be arranged that the resulting cloud of powder has no net velocity in the direction of any one nozzle and also, by suitable disposition of the nozzles that no appreciable Wave motion is set up in the longitudinal direction of the chamber liable to penetrate to and disturb the air in the lower portion of the chamber.

The particular arrangement illustrated comprises a pair of nozzle 12 and 13, connected through the valve 14 to a source of compressed air indicated diagrammatically as a bottle 15. In each lead there is provided a powder reservoir 16 and 17. It is not strictly necessary that there should be two powder reservoirs, but it is desired to mainrain as precisely as possible the balance between the air blastsand the powder issuing through the nozzles 12 and 13. By arranging the nozzles vertically as shown the net effect of the two meeting puffs of powder will tend to be a horizontal, outward flow and therewill be no wave launched vertically in the chamber tending to travel down the chamber to disturb the air in the lower part. Some turbulence will of course beset up in the region of the nozzles, but this has the advantage that the powder tends to be more evenly disposed and that the lower'nozzle 12 will not cast a shadow upon the screen at the bottom of the chamber, as would be the case if the powder particles were established in their straight vertical descent before passing it. There will be sufiicient turbulence below it to carry particles uniformly over. the whole cross section of the chamber. In operation the valve 14 is operated for a short burst so that puffs of air are released through both powder reservoirs and through both nozzles which carry a suflicient quantity of powder to provide the desired thickness of coating over the whole area of the screen The cloud thus formed then begins to settle down through the chamber. By the time the main body of the cloud is about halfway down the turbulence set up in the upper part of the chamber has produced a substantially uniform distribution of powder over the cross section of the chamber and from there on the dispersed powder falls freely in straight paths through the stagnant air of the lower part of the chamber, and through the shadow mask on to the screen surface. The screen surface may be coated with an adhesive such, for example, as silicone varnish in order to ensure a good adhesion of the particles on the surface at the moment they touch it. When the powder has settled, the screen and shadow mask assembly can be Withdrawn from the chamher and the powder which has been intercepted by the shadow mask may then be removed. The process is then repeated with the shadow mask and screen in the different attitules required for the disposition of the different phosphor strips and, of course, using different powders, so as to build up the three-colour mosaic. In general, it will of course be convenient to use a plurality of chambers, each containing the appropriate jig to position correctly the screen/shadow mask assembly for the particular phosphor to be deposited in the chamber.

In the apparatus as shown there is also provided a shutter 18 which may be inserted across the lower part of the chamber 10 just above the screen support 11. The purpose of this is to intercept any particles of unduly large size which might be released into the chamber and spoil the deposited coating. Such large particles will drop more rapidly than the very fine particles which are preferably used for the purpose of the invention and will therefore reach the shutter 18 before the main cloud. The shutter 18 may thus be rapidly withdrawn between the time when these coarse particles arrive and the arrival of the main cloud, thus removing the coarse particles before they can damage the deposit.

In fact particles size has a very great effect upon the success of the process. The particles can hardly be too small except that at an average diameter below about 5 microns they would take too longto settle and random lateral motions (Brownian motions) become significant.

If they are too large, however, they tend to drop too fast and acquire such velocity that when they strike some part of the screen structure they will tend to bounce away. As the screen is in most cases sloping, any particles which bounce are liable to bounce towards one side (down the slope) and land outside the area where the coating is required. Again, if they should strike the edge of a conductor 1 of the shadow mask they may acquire suflicient sideways momentum to fall out of place. With a shadow mask of the relative dimensions which have been mentioned above the maximum particle sizes which give suflicient freedom from this elfect for practical'purposes are in the region of 2535 microns diameter. The optimum size of particle is in the region of 5l0 microns diameter. Other dimensions may well apply to structures of different relative dimensions from those detailed above.

Many other designs of apparatus are possible and particularly is the arrangement of nozzles susceptible of variation. One alternative form of nozzle is illustrated by way of example in Fig. 4. This shows a structure in the form of a hollow mushroom mounted on the end of an inlet pipe. Arranged around the rim are nozzles 20, pointing outwardly and upwardly so as to disperse the powder all around the nozzle in an upwardly inclined direction.

In another arrangement nozzles fed from a ring manifoldv are positioned all around the wall of the chamber a short distance below the top, the nozzles protruding only slightly into the chamber, and being preferably directed slightly upwardly as in the arrangement of Fig. 4. Horizontal arrangements of opposed nozzles may also be used arranged so that the blast from one cancels the blast from another, but care must be taken that unwanted vertical waves or currents are not thereby set up.

The quantity of powder released by the mechanism into the cloud will of course depend amongst other things upon the time for which the valve 14 is open. Clearly this valve may be operated automatically, for example, electromagnetically so as to provide a carefully controlled dosage of powder on each operation of the apparatus.

It has been mentioned above that the glass surface on which the screen is to be formed by the method according to the invention may first be coated with adhesive such as silicone varnish so as to reduce the extent to which the article will rebound from or roll on the glass surface. Varnish materials which have been used are those manufactured by Midland Silicones Ltd. and marketed under the designations M. S. 996, M. S. 994 and M. S. 840. Actually the particles tend to adhere quite firmly electrostatically without the use of a special agent and the effect may be enhanced by the application of an electrostatic potential. Hence another possible technique for producing permanent adhesion of the coating on the glass comprises including with the powder a sticking agent which only becomes elfective when the structure is heated after the coating has been deposited.

Throughout the above description it has been assumed that the finely divided material of which the coating is to consist is dry powder. The use of liquid material atomised to form the cloud in the upper part of the chamber is not, however, to be excluded. If atomised liquid is to be used it must be capable of atomising sufiiciently finely to behave in the desired manner, although the different behaviour of liquid particles on impact with a solid surface tends to allow slightly more latitude in the sizes of the droplets which may be tolerated. On the other hand it may be found that the greater variation in drop size obtained with most atomising systems may make the use of shutter 18 more necessary.

Another respect in which the apparatus is susceptible of variation is in the design of chamber. Thus, in order to avoid turbulence or wave motions penetrating into the region Where vertical descent of the particles through stagnant air is essential the upper part of the chamber may be extended into cavities or fitted with baffles designed in accordance with well established aerodynamic principles to trap and/or attenuate any air disturbances set up by the cloud-forming apparatus.

Many other variations will occur to those skilled in the art.

What is claimed is:

1. Method of applying a coating of finely divided material to a surface through a stencil supported in spaced relation thereto in such a manner that the coated area conforms to a projection of the configurations of the stencil on the surface, comprising enclosing the surface to be coated and the stencil in thelower part of a substantially airtight chamber with said surface and stencil arranged in parallel relation at an inclination to the vertical corresponding to the desired angle of projection of said stencil on said surface, maintaining a column of stagant air in the lower part of said chamber, introducing said finely divided material into the upper part of said chamber in a plurality of airborne streams so directed as to form a cloud. wherein the material is distributed substantially uniformly over the cross section of the chamber above said stencil without producing any appreciable wave motion in a vertically downward direction capable of disturbing the stagnant column of air in the lower part of said chamber, and allowing the finely divided material to settle through the column of stagnant air, through the stencil and on to the surface by gravity.

2. Method as claimed in claim 1, wherein the particle size of the material is so related to the spacing of said stencil from the surface to be coated that bouncing of the particles beyond the desired boundaries of the pattern of the deposit is substantially avoided.

3. Method as claimed in claim 2 wherein the sizes of the particles of said coating material are in the region of from 5 to 35 microns in diameter when the spacing between the stencil and the surface to be coated is on the order of 0.024 inch.

4. Apparatus for applying coating material in finely divided state to a surface through a stencil supported in spaced relation thereto comprising a substantially airtight, vertically elongated chamber having a column of stagnant air in the lower part thereof, means within said chamber near its lower end for supporting said surface and said stencil in a desired attitude towards the vertical, means for injecting said coating material in fine ly divided state into the upper part of said chamber as an airborne cloud, and means for neutralising the momentum of the carrier air stream so as to avoid the production of turbulence in the stagnant air column in the lower part of said chamber.

5. Apparatus as claimed in claim 4 wherein said injecting means comprise a plurality of nozzles and means for feeding said coating material to said nozzles.

6. Apparatus as claimed in claim 5 wherein said nozzles are directed in mutually opposed directions towards one another whereby jets issuing from said nozzles tend to destroy each others momentum.

7. Apparatus as claimed in claim 5 including at least one reservoir for said coating material and means for blowing compressed air through said reservoir to said nozzles to set up said cloud.

8. Apparatus as claimed in claim 4, including a movable shutter in said chamber between said injecting means and said supporting means, adapted at times to intercept particles of coating material falling through said chamber, and at other times to be withdrawn from the path of particles falling through said chamber.

9. Apparatus for depositing coatings of finely divided powder through a stencil on to a surface comprising a substantially air-tight, vertically elongated chamber, means at the lower end of said chamber for supporting said surface and said stencil With the stencil in spaced relation to and above said surface, a pair of nozzles, one above the other facing one another, disposed high up in said chamber, ducts connected said nozzles each to a reservoir for said powder and said reservoirs to a source of pressure gas, and a movable shutter adapted at times to be introduced across said chamber below said nozzles to intercept particles falling towards said surface and to be withdrawn from the path of said particles at other times.

References Cited in the file of this patent UNITED STATES PATENTS 2,097,233 Meston Oct. 26, 1937 2,682,478 Howse June 29, 1954 2,690,979 Law Oct. 5, 1954 2,727,828 Law Dec. 20, 1955 Smith Jan. 31, 1956 U UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No, 2,872,339 February 3, 1959 Dennis Gabor et a1,

It is hereby certified that error appears in the printed specification of the above numbered patent requiring correction and that the said Letters Patent should read as corrected below.

Column 1, line 38, strike out "and now U, S. Patent No 2,794,655,"; column 5, line 25 for "nozzle read m1 nozzles line 67, for attitule: read attitudes column 6, line 12, for "particles" read m particle= column '7, line 45, after claim l" strike out the comma; column 8, line 25, after "claim 4" strike out the comma,

Signed and sealed this 9th day of June i959 5m) ttest:

ROBERT C. WATSON KARL Ho AXLINE Commissioner of Patent Attesting Oflicer 

1. METHOD OF APPLYING A COATING OF FINELY DIVIDED MATERIAL TO A SURFACE THROUGH A STENCIL SUPPORTED IN SPACED RELATION THERETO IN SUCH A MANNER THAT THE COATED AREA CONFORMS TO A PROJECTION OF THE CONFIGURATIONS OF THE STENCIL ON THE SURFACE, COMPRISING ENCLOSING THE SURFACE TO BE COATED AND THE STENCIL IN THE LOWER PART OF A SUBSTANTIALLY AIRTIGHT CHAMBER WITH SAID SURFACE AND STENCIL ARANGED IN PARALLEL RELATION AT AN INCLINATION TO THE VERTICAL CORRESPONDING TO THE DESIRED ANGLE OF PROJECTION OF SAID STENCIL ON SAID SURFACE, MAINTAINING A COLUMN OF STAGANT AIR IN THE LOWER PART OF SAID CHAMBER, INTRODUCING SAID FINELY DIVIDED MATERIAL INTO THE UPPER PART OF SAID CHAMBER IN A PLURALITY OF AIRBOURNE STREAMS SO DIRECTED AS TO FORM A CLOUD WHEREIN THE MATERIAL IS DISTRIBUTED SUBSTANTIALLY UNIFORMLY OVER THE CROSS SECTION OF THE CHAMBER ABOVE SAID STENCIL WITHOUT PRODUCING ANY APPRECIABLE WAVE MOTION IN A VERTICALLY DOWNWARD DIRECTION CAPABLE OF DISTURBING THE STAGNANT COLUMN OF AIR IN THE LOWER PART OF SAID CHAMBER, AND ALLOWING THE FINELY DIVIDED MATERIAL TO SETTLE THROUGH THE COLUMN OF STAGNANT AIR, THROUGH THE STENCIL AND ON TO THE SURFACE BY GRAVITY. 